“Common carrier” law does scale, but the entertainment industry (or the publishers and investors behind it, at least) has never been a fan of the notion. After all, common carriers have virtually no duties to industry, apart from dealing fairly and evenly with all comers. If Warner Music Group called up AT&T to complain that someone in Des Moines was holding up their phone to a boom box and transmitting copyrighted music to a confederate in Arroyo Alto, Texas (population 363), AT&T could shrug its mighty shoulders and turn away.
The Goldilocks middle ground here is Notice and Takedown (NaTD)—the other part of the WIPO Copyright Treaty. Notice and Takedown works pretty much like it sounds: if you think a file hosted on my server is infringing on your copyright, you tell me (Notice) and I have to remove it (Takedown). As long as I do so, I’m considered a common carrier—no matter what happens in the ensuing legal battle, I’m not liable for it. If I don’t take it down, I’m in strict-liability land: if you win a judgment against the guy who’s using my server to infringe on your copyright, you can sue me for damages.
If there’s one word that comes up consistently in discussions of NaTD, it’s “balance.” As in, “This is a way to balance the rights of creators and investors with the business realities of online media.”